Two species of alpha-Lactalbumin (alpha LA) have been found in rat milk and both are active in a lactose synthetase assay. The two alpha LA's differ in apparent molecular weights (22,500 and 21,800 daltons respectively) and net negative charge. A radioimmunoassay (RIA) was developed and used to detect alpha LA in rat mammary carcinomas. In this assay unlabeled antigen competed against 125I labeled antigen for binding to antibody against pure rat alpha LA of low molecular weight. However, the two alpha-lactalbumins are indistinguishable by the assay which is able to detect 3-5 nanograms of alpha LA. Primary carcinogens induced in rats by 7-12 Dimethylbenzanthracene (DMBA) or methylnitrosourea (MNU) contained alpha LA in quantities equal to or less than 10% the amounts found in 3-5 day rat lactating mammary gland. Alpha LA was found only in R3230 AC and MTW9 but not in DMBA #1, Ascites 13762 and MTW9A transplantable mammary carcinomas. Transplantation of a pituitary gland under the kidney capsule of the host (a) increased alpha LA content of primary DMBA induced mammary carcinomas, (b) reduced alpha LA content of primary MNU induced mammary carcinomas and (c) was unable to modify the alpha LA levels in 5 transplantable mammary carcinomas.